We report our identification of the optical afterglow and host galaxy of the short-duration gammaray burst sGRB 160821B. The spectroscopic redshift of the host is z = 0:162, making it one of the lowest redshift sGRBs identified by Swift. Our intensive follow-up campaign using a range of groundbased facilities as well as HST, XMM-Newton and Swift, shows evidence for a late-time excess of optical and near-infrared emission in addition to a complex afterglow. The afterglow light-curve at X-ray frequencies reveals a narrow jet, ∼ 1.9+0.10-0.03 deg, that is refreshed at >1 day post-burst by a slower outow with significantly more energy than the initial outow that produced the main GRB. Observations of the 5 GHz radio afterglow shows a reverse shock into a mildly magnetised shell. The optical and near-infrared excess is fainter than AT2017gfo associated with GW170817, and is well explained by a kilonova with dynamic ejecta mass Mdyn = (1.0-0.6) × 10-3Mo and a secular (postmerger) ejecta mass with Mpm = (1.0-0.6) × 10-2Mo, consistent with a binary neutron star merger resulting in a short-lived massive neutron star. This optical and near-infrared dataset provides the best-sampled kilonova light-curve without a gravitational wave trigger to date.
|State||Published - May 6 2019|
- (Stars:) gamma-ray burst: Individual GRB 160821B
- stars: Neutron